STATS completes Asian subsea isolation

STATS Group has completed a temporary isolation on a subsea gas export line in the South China Sea.

The company was contracted by COOEC Subsea Technology to
complete the work on the Yacheng pipeline, a 780 km, 28 inch (711 mm) subsea
gas export line that supplies gas to Hong Kong.

STATS was called in
after an anchor damaged one of two subsea pipeline end manifolds (PLEMs) and
left a severe dent leaving it unpiggable.

The damaged section was situated approximately 280 km from
Hong Kong at a depth of 90 m.

COOEC initially implemented a temporary repair, installing a
400 m, 14 inch (356 mm) bypass between the PLEMs, which enabled the pipeline to
continue operating at 70 bar while the permanent repair method took place.

The repair was completed by recovering the subsea pipeline
from the seabed onto a pipelay vessel, while the pipeline remained pressurised
at a shut-in pressure of 57 bar; it included the removal of the severely dented
pipeline and both existing PLEMs and the 400 m of pipeline between them.

Subsea flangeless plug launcher. Image courtesy STATS.

The pipeline’s crucial importance to gas supply to Hong Kong
meant the shutdown period needed to be kept to a minimum and the repair completed
without depressurising and flooding the entire line.

To ensure recovery of both ends of the pipeline onto the
pipelay vessel without depressurisation or water ingress, STATS Tecno Plug™ and
BISEP isolation tools were installed, enabling isolation of only the sections
of pipeline being recovered from the seabed.

The isolation tools had to provide fully proved, fail-safe,
double block and bleed isolation barriers from the pressurised pipelines,
complying with industry guidance on isolation and intervention for diver access
to subsea systems.

As a result of the defect, which made the pipeline
unpiggable, it was not possible to pig isolation plugs through the pipeline
from the permanent pig launcher, so STATS proposed the use of its dual seal
BISEP isolation plug, which is installed at location through a hot tap clamp
fitting.

It was deployed into the pressurised pipeline downstream of
the damaged pipeline and provided a fully proved double block and bleed
isolation barrier against the pipeline gas pressure, allowing the pipeline to
be depressurised between the rear of the BISEP and the PLEM while the damaged
section of pipeline was safely cut and removed.

With access to the pipeline gained, STATS’ dual seal
flangeless plug launcher was attached to the bare pipe end allowing two
remotely operated Tecno Plugs to be pigged into the pipeline.

The BISEP could then be unset and recovered, allowing the
first Tecno Plug to be pigged 700 m into the pipeline and set, providing
isolation of the pipeline gas pressure.

Image courtesy STATS.

The second plug was then pigged into the pipeline and set a
short distance downstream of the hot tap fitting; this allowed the pipeline to
be cut behind the rear Tecno Plug and the removal of the BISEP, fittings and
hot tap penetration.

At the opposite end of the pipeline, the PLEM provided
double block isolation allowing the flangeless plug launcher to be attached to
the open pipe end and another two Remote Tecno Plugs were pigged into the line
700 m apart.

Once both plugs were set and double block isolation was
confirmed, the isolation certificate was issued and the second PLEM was removed
and recovered to the vessel.

A pipeline retrieval tool was then used to recover the
pipeline to the pipelay vessel allowing new pipeline sections to be welded to
the existing pipeline before laying them back onto the seabed via a flanged
laydown head.

The four Tecno Plugs in the pipeline were continuously remotely
monitored from a diver support vessel while the pipeline was being isolated.

To prevent seawater flooding into the newly laid pipeline
sections when connecting the pipeline to the new PLEMs, the rear plugs were
repositioned by equalising the pressure, unsetting them and pigging them back
to the newly attached laydown heads.

The plugs were then reset and their double block isolations
were proved again so that the laydown heads could be safely removed by the
divers.

Once the PLEMs were connected to the new pipeline, subsea
receivers were fitted and used to recover both sets of Tecno Plugs out of each
pipeline section.

Pipeline end manifold Deployment. Image courtesy STATS.

After all of the plugs were confirmed to be in the subsea
receivers, the subsea pipeline isolation valves in the PLEMs were closed and
the receivers were recovered to the vessel.

The repair was completed by connecting the two PLEMs
together with a final 25 m tie-in spool, which was then leak tested and
dewatered.

The subsea pipeline isolation valves were then opened and
the critical gas supply to Hong Kong was restored.

According to STATS, the pipeline was repaired as safely and
quickly as possible, without depressurising or flooding the pipeline and ensuring
that any seawater that may have entered the system was removed before the
pipeline resumed operation.